Conventional means for delivering active agents to their intended targets, e.g. human organs, tumor cites, etc., are often severely limited by the presence of biological, chemical, and physical barriers. Typically, these barriers are imposed by the environment through which delivery must take place, the environment of the target for delivery, or the target itself.
Biologically active agents are particularly vulnerable to such barriers. Oral delivery to the circulatory system for many biologically active agents would be the route of choice for administration to animals if not for physical barriers such as the skin, lipid bi-layers, and various organ membranes that are relatively impermeable to certain biologically active agents, but which must be traversed before an agent delivered via the oral route can reach the circulatory system. Additionally, oral delivery is impeded by chemical barriers such as the varying pH in the gastrointestinal (GI) tract and the presence in the oral cavity and the GI tract of powerful digestive enzymes.
Calcitonin and insulin exemplify the problems confronted in the art in designing an effective oral drug delivery system. The medicinal properties of calcitonin and insulin can be readily altered using any number of techniques, but their physicochemical properties and susceptibility to enzymatic digestion have precluded the design of a commercially viable delivery system. Others among the numerous agents which are not typically amenable to oral administration are biologically active proteins such as the cytokines (e.g. interferons, IL-2, etc); erythropoietin; polysaccharides, and in particular mucopolysaccharides including, but not limited to, heparin; heparinoids; antibiotics; and other organic substances. These agents are also rapidly rendered ineffective or are destroyed in the GI tract by acid hydrolysis, enzymes, or the like.
Biotechnology has allowed the creation of numerous other compounds, of which many are in clinical use around the world. Yet, the current mode of administration of these compounds remains almost exclusively via injection. While in many cases oral administration of these compounds would be preferable, these agents are labile to various enzymes and variations in pH in the GI tract and are generally unable to penetrate adequately the lipid bilayers of which cell membranes are typically composed. Consequently, the active agent cannot be delivered orally to the target at which the active agent renders its intended biological effect.
Typically, the initial focus of drug design is on the physiochemical properties of pharmaceutical compounds and particularly their therapeutic function. The secondary design focus is on the need to deliver the active agent to its biological target(s). This is particularly true for drugs and other biologically active agents that are designed for oral administration to humans and other animals. However, thousands of therapeutic compounds are discarded because no delivery systems are available to ensure that therapeutic titers of the compounds will reach the appropriate anatomical location or compartment(s) after administration and particularly oral administration. Furthermore, many existing therapeutic agents are underutilized for their approved indications because of constraints on their mode(s) of administration. Additionally, many therapeutic agents could be effective for additional clinical indications beyond those for which they are already employed if there existed a practical methodology to deliver them in appropriate quantities to the appropriate biological targets.
Although nature has achieved successful inter- and intra-cellular transport of active agents such as proteins, this success has not been translated to drug design. In nature, the transportable conformation of an active agent such as a protein is different than the conformation of the protein in its native state. In addition, natural transport systems often effect a return to the native state of the protein subsequent to transport. When proteins are synthesized by ribosomes, they are shuttled to the appropriate cellular organelle by a variety of mechanisms e.g. signal peptides and/or chaperonins. Gething, M-J., Sambrook, J., Nature, 355, 1992, 33-45. One of the many functions of either the signal peptides or the chaperonins is to prevent premature folding of the protein into the native state. The native state is usually described as the 3-dimensional state with the lowest free energy. By maintaining the protein in a partially unfolded state, the signal peptides or the chaperonins facilitate the protein's ability to cross various cellular membranes until the protein reaches the appropriate organelle. The chaperonin then separates from the protein or the signal peptide is cleaved from the protein, allowing the protein to fold to the native state. It is well known that the ability of the protein to transit cellular membranes is at least partly a consequence of being in a partially unfolded state.
Current concepts of protein folding suggest that there are a number of discrete conformations in the transition from the native state to the fully denatured state. Baker, D., Agard, D. A., Biochemistry, 33, 1994, 7505-7509. The framework model of protein folding suggests that in the initial early stages of folding the domains of the protein that are the secondary structure units will form followed by the final folding into the native state. Kim, P. S., Baldwin, R. L., Annu. Rev. Biochem., 59, 1990, 631-660. In addition to these kinetic intermediates, equilibrium intermediates appear to be significant for a number of cellular functions. Bychkova, V. E., Berni, R., et al, Biochemistry, 31, 1992, 7566-7571, and Sinev, M. A., Razgulyaev, O. I., et al, Eur. J. Biochem., 1989, 180, 61-66. Available data on chaperonins indicate that they function, in part, by keeping proteins in a conformation that is not the native state. In addition, it has been demonstrated that proteins in partially unfolded states are able to pass through membranes, whereas the native state, especially of large globular proteins, penetrates membranes poorly, if at all. Haynie, D. T., Freire, E., Proteins:Structure, Function and Genetics, 16, 1993, 115-140.
Similarly, some ligands such as insulin which are unable to undergo conformational changes associated with the equilibrium intermediates described above, lose their functionality. Hua, Q. X., Ladbury, J. E., Weiss, M. A., Biochemistry, 1993, 32, 1433-1442; Remington, S., Wiegand, G., Huber, R., 1982, 158, 111-152; Hua, Q. X., Shoelson, S. E., Kochoyan, M. Weiss, M. A., Nature, 1991, 354, 238-241.
Studies with diphtheria toxin and cholera toxin indicate that after diphtheria toxin binds to its cellular receptor, it is endocytosed, and while in this endocytic vesicle, it is exposed to an acidic pH environment. The acidic pH induces a structural change in the toxin molecule which provides the driving force for membrane insertion and translocation to the cytosol. See, Ramsay, G., Freire, E. Biochemistry, 1990, 29, 8677-8683 and Schon, A., Freire, E., Biochemistry, 1989, 28, 5019-5024. Similarly, cholera toxin undergoes a conformational change subsequent to endocytosis which allows the molecule to penetrate the nuclear membrane. See also, Morin, P. E., Diggs, D., Freire, E., Biochemistry, 1990, 29, 781-788.
Earlier designed delivery systems have used either an indirect or a direct approach to delivery. The indirect approach seeks to protect the drug from a hostile environment. Examples are enteric coatings, liposomes, microspheres, microcapsules. See, colloidal drug delivery systems, 1994, ed. Jorg Freuter, Marcel Dekker, Inc.; U.S. Pat. No. 4,239,754; Patel et al. (1976), FEBS Letters, Vol. 62, pg. 60; and Hashimoto et al. (1979), Endocrinology Japan, Vol. 26, pg. 337. All of these approaches are indirect in that their design rationale is not directed to the drug, but rather is directed to protecting against the environment through which the drug must pass enroute to the target at which it will exert its biological activity, i.e. to prevent the hostile environment from contacting and destroying the drug.
The direct approach is based upon forming covalent linkages with the drug and a modifier, such as the creation of a prodrug. Balant, L. P., Doelker, E., Buri, P., Eur. J. Drug Metab. And Pharmacokinetics, 1990, 15(2), 143-153. The linkage is usually designed to be broken under defined circumstances, e.g. pH changes or exposure to specific enzymes. The covalent linkage of the drug to a modifier essentially creates a new molecule with new properties such as an altered log P value and/or as well as a new spatial configuration. The new molecule has different solubility properties and is less susceptible to enzymatic digestion. An example of this type of method is the covalent linkage of polyethylene glycol to proteins. Abuchowski, A., Van Es, T., Palczuk, N. C., Davis, F. F., J. Biol. Chem. 1977, 252, 3578.
Broad spectrum use of prior delivery systems has been precluded, however, because: (1) the systems require toxic amounts of adjuvants or inhibitors; (2) suitable low molecular weight cargos, i.e. active agents, are not available; (3) the systems exhibit poor stability and inadequate shelf life; (4) the systems are difficult to manufacture; (5) the systems fail to protect the active agent (cargo); (6) the systems adversely alter the active agent; or (7) the systems fail to allow or promote absorption of the active agent.
There is still a need in the art for simple, inexpensive delivery systems which are easily prepared and which can deliver a broad range of active agents to their intended targets, expecially in the case of pharmaceutical agents that are to be administered via the oral route.